لغة الجسد مهمة.

A natural classroom-style pronunciation is:

lughat al-jasad muhimma

A few notes:

• لغة on its own is usually pronounced lugha.
• But here it is followed by another noun in a construct phrase, so the final ة is pronounced like t: lughat.
• مهمة is usually pronounced muhimma at the end of the sentence.

If you read it with full case endings in very careful MSA, it would be:

lughatu l-jasadi muhimmatun

Word by word:

• لغة = language
• الجسد = the body
• مهمة = important

But لغة الجسد is not just language + body separately. Together, they form the expression body language.

So the structure is:

• لغة الجسد = body language
• مهمة = important

Because in Arabic, present-tense sentences like X is Y usually do not use a verb equivalent to is.

So:

• لغة الجسد مهمة literally looks like body language important
• But the correct English meaning is Body language is important

This is called a nominal sentence in Arabic.

If you wanted the past tense, then Arabic would use a verb, for example:

• كانت لغة الجسد مهمة = Body language was important

This is because لغة الجسد is an iḍāfa construction, often called a construct phrase.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun can take ال
• the whole phrase can become definite because of the second noun

So:

• لغة الجسد literally = the language of the body
• natural English = body language

You would not normally say اللغة الجسد here.

Because لغة ends in ة (tāʾ marbūṭa).

A tāʾ marbūṭa is often pronounced:

• as -a / -ah when the word is said by itself or at a pause
• as t when followed by another word in grammar-connected speech

So:

• by itself: لغة → lugha
• in this phrase: لغة الجسد → lughat al-jasad

This is very common in Arabic.

Because لغة is a feminine noun, and the predicate adjective must agree with it in gender.

So:

• لغة = feminine
• therefore important must also be feminine: مهمة

Compare:

• masculine: الكتاب مهم = The book is important
• feminine: اللغة مهمة = The language is important

So in your sentence, مهمة is feminine because it is describing لغة الجسد.

A very important clue is definiteness.

• لغة الجسد is a definite noun phrase
• مهمة is indefinite

In Arabic, if an adjective directly modifies a noun, it usually matches it in definiteness.

So:

• لغة الجسد مهمة = Body language is important
  • here مهمة is the predicate
• لغة الجسد المهمة = the important body language
  • here المهمة is an adjective modifying the noun phrase

So the lack of ال on مهمة helps show that this is a full sentence, not just a noun phrase.

In fully vowelled MSA, the sentence is:

لُغَةُ الجَسَدِ مُهِمَّةٌ

Here is what the endings show:

• لغةُ = nominative, because it is the subject of the nominal sentence
• الجسدِ = genitive, because it is the second term of the iḍāfa
• مهمةٌ = nominative, because it is the predicate

In everyday reading and speaking, these short ending vowels are often not fully pronounced, but in formal grammar they are there.

Because it is the second noun in an iḍāfa.

In Arabic, the second part of an iḍāfa is always in the genitive case.

So:

• لغةُ = language
• الجسدِ = of the body

Together:

• لغةُ الجسدِ = the language of the body

That is why الجسد takes the genitive ending in fully vowelled Arabic.